Crewed Mission to Mars--Jan. 2018

That's actually a consideration, and one that's quite valid. I don't see many companies (with any significant assets/budget) buying ad space on the side of something that's more likely than not to become a perpetual orbiting tomb. And that's before we get to the question of, "what's to be gained here?" The answer to which appears to be, "Not much". At least the Shuttle was up there doing actual, useful work.

Red Bull is plenty happy to put their name on things that have a high risk of killing people, and I'm not even talking about the stuff you mix with Jaegermeister.

What kind of shielding would be required to mitigate that kind of flare?

Ideally you'd want as much of the densest stuff you could get between the crew and 'outdoors'. Lead, steel, concrete, packed soil, etc.

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Is it impractical?

That's the main problem. It's like trying to armor any flying vehicle - it's all about compromise. It's possible to armor it sufficiently to deal with any reasonably imagined threat, but at some point it's just going to be too heavy to fly. So the operative question here is, "Can it be shielded well enough?"

Correct. For passive shielding, density is king. Active shielding can be employed as well (in theory) in the form of a generated magnetic field around the spacecraft, but that obviously has power and complexity implications associated with it. Also, due to the nature of the generated field, you'll have areas that are well protected along with a couple of poles that effectively funnel a concentrated amount of radiation into specific points on the spacecraft.

In short, there's no perfect, catch-all solution, so a layered combination of methods will probably be needed/most effective. I'd probably approach it in a manner similar to the one used for the Iowa class battleships - anything that was absolutely vital was protected, anything that wasn't absolutely critical wasn't.

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The idea of using the consumables storage as an impromptu radiation shield is interesting, but would it have the density?

Water is surprisingly good at the job. Not the best, but you're going to need it anyway, so making it pull double duty as a passive radiation shield certainly has its upsides. A sheet of lead would provide better protection, but is otherwise dead-weight. Normally what's done is certain areas of larger spacecraft are designated radiation shelters, and built to provide greater protection than the rest of the structure. In times of increased radiation risk, the crew retreats to those areas for the duration.

In manned orbital flights the SAA must be considered, but I expect even it is fairly tame compared to what you'd potentially encounter in interplanetary space.

IMO, all of this just further points to how much harder this is than just putting people in orbit for a year, or sending them on a trip to the Moon. I don't think most people really appreciate how much of a step up in difficulty a trip to Mars is, let alone a manned trip.

I don't doubt that within five years and a massive stretching of the proposed hardware that it may be possible to fire a couple of people around Mars and back again as live cargo, but I have serious doubts that it could be done with any significant chance of ensuring that cargo's health/survival. And even then, at best it'd be a "one off" stunt, as that's certainly not a good way to attempt the trip going forward. It's just not a viable system.

Which is why ultimately, I think this is at best a pipe dream, and at worst, a good way to waste a couple of lives and a lot of resources that could have been put to better use obtaining a viable, longer term contribution to the overall goal.

In short: Are you more interested in a short term, high risk stunt for show, or would you rather work on making the future a reality?

If we're going to kill people in the attempt either way (and make no mistake, people will die going to Mars), I know which I'd rather it was for.

Red Bull is plenty happy to put their name on things that have a high risk of killing people, and I'm not even talking about the stuff you mix with Jaegermeister.

Imagine the marketing possibilities, they could bring a one year supply of Red Bull with them. They could even bring an inflatable Red Bull can and put it in orbit around Mars when they do the fly by, that in itself would be worth a hundred million.

Only "passive shielding", i.e. packing lots of dense material between you and space works. Even then, over time, the shielding itself becomes radioactive and degraded chemically due to nuclear reactions caused by cosmic rays. A magnetic field shield won't protect you from higher energy charged particles, since they're too high energy to be deflected much. Nor will it protect you from uncharged radiation, like gamma rays.

Long term, if humans are ever to go into deep space (why?), it's all about (1) genetically engineering human biology to be extremely resistant to radiation and DNA damage (2) having a cure for any cancer.

New Scientist reports that part of the radiation shielding plan is to keep a 40cm thick layer of water and food in bags along the capsule walls, then - to put it as delicately as I can - to put them back there once the crew are done with them. It also points out that they'll be flying around with the empty second stage attached, which could then be oriented towards the Sun in case it decides to give them a good storm.

Using the TVT figures from Project Rho, a 40cm water layer should be able to screen out 97.5% of particle radiation and 78% of gamma rays.

Falcon 9's second stage is 4.7 tons of dry mass, 3.66m diameter, so if that's steely it'd have a mean thickness of 5.5cm and get rid of 69% of gamma rays. Sit the crew on top of the oxygen supply, CO2 scrubbers etc too.

Which is why I'm wondering why the benefactor here is throwing all this money at a stunt, when he could be putting it toward research that could make manned Mars trips much more practical.

Yep. There are so many better places to drop this much cash that it's hard to get behind something like this. I really don't expect them to actually put a real effort into this, though at the moment I'm at a loss as to why they'd try for a pure attention grab at this time.

Want to show you're serious? Start with a Moon mission. Yeah, sure, been done. But not in 40 years, and not with currently available technology. Meanwhile work on developing that necessary, non-chemical rocket based propulsion system for your 'real' objective of Mars. The moonshot would gain valuable practical experience and start training your people (engineers/support etc.) to start thinking and planning as they'll need to for Mars duration flight, and the setting of a difficult but more reasonably attainable (and far less risky) initial goal grants a much higher chance of success and follow-on support.

As it stands it seems this project wants to jump from tooling around on a Big Wheel to directly onto a high performance motorcycle on the professional racing circuit with no steps in-between. Maybe they'll get spectacularly lucky, but I wouldn't bet a dime on it.

This is the bit I don't get.

Surely it makes more sense to go for a far more marketable Moon tourism effort to let wealthy people do a low altitude orbit of the Moon, see Earthrise and perhaps even do a landing and use that to develop the technology that would be used for a Mars shot. Sending two oldies on a multi-billion dollar 500 day suicide attempt seems like a much worse way of encouraging the commercialisation of space than going for the simpler option of a modern Moon landing that lets billionaires live out their Apollo astronaut fantasies.

This isn't just running before you can walk, it's flying before you can crawl.

New Scientist reports that part of the radiation shielding plan is to keep a 40cm thick layer of water and food in bags along the capsule walls, then - to put it as delicately as I can - to put them back there once the crew are done with them. It also points out that they'll be flying around with the empty second stage attached, which could then be oriented towards the Sun in case it decides to give them a good storm.

New Scientist reports that part of the radiation shielding plan is to keep a 40cm thick layer of water and food in bags along the capsule walls, then - to put it as delicately as I can - to put them back there once the crew are done with them. It also points out that they'll be flying around with the empty second stage attached, which could then be oriented towards the Sun in case it decides to give them a good storm.

That actually makes a lot of sense.

It would suck to grab the wrong bag at dinnertime, though.

"Awwww, chocolate pudding again? Hon, is it just me, or has all this time in zero-g ruined my sense of taste? This pudding has tasted like shit for months now."

If you use water for shielding, would it still be safe to drink that water?

Yes. Very much yes. You would have water with a tiny sprinkling of mostly hydrogen dissolved in it, ie. it would be slightly acidic. Also, there would be even smaller amounts of heavier elements like carbon, oxygen, and iron dissolved in. Essentially nothing heavier than that. Perfectly 100% safe.

Also, no, cosmic radiation doesn't activate materials like reactor neutron radiation does. The flux is far too low and realistic mission durations (ie., around 3 years max for the full landing mission) much too short for any real effect. Think of it this way: Moon rocks, which have been exposed to cosmic radiation with absolutely no shielding whatsoever for millions or even billions of years are not dangerously radioactive compare to Earth rocks. Given that space vehicles will be exposed for far shorter periods of time, why should they become radioactive?

Furthermore, passive shielding is effective, but only up to a certain point, at least against GCRs (which are the only real radiation threat worth worrying about to any great extent; solar protons, while high-flux during flare events, are much lower energy and easier to shield against). And lighter, more hydrogenic (ie., ones with more hydrogen) materials are always better for protection against cosmogenic radiation (on a per-weight basis), particularly as they reduce bremsstrahlung secondaries. Liquid hydrogen is just about the best radiation protection you can get, but it's kind of cold; plastics and water are almost as good but obviously more compatible with normal life-support requirements. Unfortunately, as I was saying, once you get to a certain point (somewhere about 20 grams per square centimeter of material, maybe 30) significantly increasing shielding mass does very little to further reduce radiation exposure.

This, by the way, is why you want to travel during a solar maximum, not a minimum. The solar magnetic field significantly decreases GCR flux (by a factor of ~33-50%), while your plastic/water GCR shielding is perfectly effective at handling solar flares with a storm cellar inside.

EDIT: By tiny, I mean that the solar wind consists of about 4.6*10^12 particles per square meter at one astronomical unit. Assuming that every single one is absorbed by the radiation shielding, and that all of them are hydrogen (the latter of which is fairly reasonable), you get a grand total of 0.722 milligrams of hydrogen deposited in your radiation shielding over three years. Even assuming that the solar wind is pure iron (which is obviously absurd), you add a mere 40 milligrams of material to the shielding. Assuming the human in question was jammed in a capsule only just about big enough to fit him or her (a cylinder two meters long and one meter in diameter), and provided 20 grams per cubic centimeter of water as per the below discussion, they are shielded by about seven hundred kilograms of water, or about twenty million times as much material.

Surely it makes more sense to go for a far more marketable Moon tourism effort to let wealthy people do a low altitude orbit of the Moon, see Earthrise and perhaps even do a landing and use that to develop the technology that would be used for a Mars shot. Sending two oldies on a multi-billion dollar 500 day suicide attempt seems like a much worse way of encouraging the commercialisation of space than going for the simpler option of a modern Moon landing that lets billionaires live out their Apollo astronaut fantasies.

This isn't just running before you can walk, it's flying before you can crawl.

This isn't just running before you can walk, it's flying before you can crawl.

I don't buy this line of criticism. In the most recent Mars mission, the skycrane was though of as incredibly risky, never been tried, only worked in theory, yet it worked as expected. There is no reason why this Mars mission cannot be executed with similar precision, accounting for all factors and possible threats.

The most notable reason there is a difference is that people are onboard.

While that does make it difficult on the life-support end of things, this mission is also just a flyby of the planet, so there are no difficult surface landing requirements or even orbital insertion problems. That simplifies things considerably.

The most notable reason there is a difference is that people are onboard.

While that does make it difficult on the life-support end of things, this mission is also just a flyby of the planet, so there are no difficult surface landing requirements or even orbital insertion problems. That simplifies things considerably.

This makes it difficult on the life-support end of things in the same way that adding a pilot is difficult on the control side of things for a 15 kg drone.

This isn't just running before you can walk, it's flying before you can crawl.

I don't buy this line of criticism. In the most recent Mars mission, the skycrane was though of as incredibly risky, never been tried, only worked in theory, yet it worked as expected. There is no reason why this Mars mission cannot be executed with similar precision, accounting for all factors and possible threats.

1) That was NASA. They kind of have a track record for making space stuff work.2) There were no human lives at risk

This isn't just running before you can walk, it's flying before you can crawl.

I don't buy this line of criticism. In the most recent Mars mission, the skycrane was though of as incredibly risky, never been tried, only worked in theory, yet it worked as expected. There is no reason why this Mars mission cannot be executed with similar precision, accounting for all factors and possible threats.

The challenge is completely different, and this one is much, much harder. Keeping people alive and healthy and sane in a floating outhouse for a year and a half is a much bigger challenge than I think many people realize.

The challenge is completely different, and this one is much, much harder. Keeping people alive and healthy and sane in a floating outhouse for a year and a half is a much bigger challenge than I think many people realize.

Which is why we circle back around to the position that spending this money researching faster ways to get there is a much better use of it than the current idea. Perhaps we need a new thread, as I find the discussion of how we get there faster to be a lot more interesting than how we get there now.

For example, there are several good articles up on Ion MPD thrusters, which have both the potential thrust and top speed to make a Mars mission a matter of a several months. The issues around using them amount to some evolutionary technical improvements for long-term reliability, and power. What kinds of issues need to be surmounted to get a nuclear reactor, similar perhaps to what's in current nuclear submarines, into a spacecraft? From a laymans perspective this doesn't sound too terribly difficult since we've been putting them in pressure hulls in extremely inhospitable environments for decades, but making sure one can run for a year or more with virtually no maintenance might be a trick. From what I can read on MPD thrusters you also don't need the 20+ megawatts of power such a plant produces... 1-2 MW is more than enough.

Falcon 9's second stage is 4.7 tons of dry mass, 3.66m diameter, so if that's steely it'd have a mean thickness of 5.5cm and get rid of 69% of gamma rays. Sit the crew on top of the oxygen supply, CO2 scrubbers etc too.

The mass wouldn't be evenly distributed, a lot of it would be in the cylinder walls, which probably doesn't help so much, and the engine, which probably does help.

Red Bull is plenty happy to put their name on things that have a high risk of killing people, and I'm not even talking about the stuff you mix with Jaegermeister.

I'm not aware of Red Bull sponsoring anything where untrained, non-professionals with money can just buy a ticket for a ride on a system that has a high probability of killing them.

Well, if you have money to buy skates, the only barrier to entry for Crashed Ice appears to be a willingness to waive the company of any responsibility and the ability to find someone else to insure your life. Read into that what you will.

I know nothing about aerospace engineering, but why couldn't they create an artificial magnetic field around the capsule to reduce/eliminate the cosmic ray threat to the crew? Is the energy required not feasible?

I know nothing about aerospace engineering, but why couldn't they create an artificial magnetic field around the capsule to reduce/eliminate the cosmic ray threat to the crew? Is the energy required not feasible?

How big a fan would you need to reduce the cannonball threat to your galleon?

I know nothing about aerospace engineering, but why couldn't they create an artificial magnetic field around the capsule to reduce/eliminate the cosmic ray threat to the crew? Is the energy required not feasible?

Uncharged particles (e.g. gamma rays) tend to be the ones that are the most dangerous, because they represent whole-body doses. Those can't be altered by magnetic fields.

Uncharged particles (e.g. gamma rays) tend to be the ones that are the most dangerous, because they represent whole-body doses. Those can't be altered by magnetic fields.

No. The vast majority of cosmic rays are high energy charged particles, mostly protons and other atomic nuclei, with gamma rays being a negligible part of the whole. If you could reduce the dose to just gammmas via electromagnetic means, then astronauts would be perfectly safe, and in fact probably receiving a lower dose than on Earth's surface. Therefore, electrostatic and electromagnetic shielding has been studied off and on since the '60s; the difficulty is that a powerful (read: energy-hungry) field is needed to deflect the high-energy particles and that particles of the "wrong" charge (electrons, basically) may not behave as desired.

NervousEnergy wrote:

For example, there are several good articles up on Ion MPD thrusters, which have both the potential thrust and top speed to make a Mars mission a matter of a several months. The issues around using them amount to some evolutionary technical improvements for long-term reliability, and power. What kinds of issues need to be surmounted to get a nuclear reactor, similar perhaps to what's in current nuclear submarines, into a spacecraft? From a laymans perspective this doesn't sound too terribly difficult since we've been putting them in pressure hulls in extremely inhospitable environments for decades, but making sure one can run for a year or more with virtually no maintenance might be a trick. From what I can read on MPD thrusters you also don't need the 20+ megawatts of power such a plant produces... 1-2 MW is more than enough.

What else do we need?

Well, a magic wand to make launching nuclear reactors into space politically acceptable, for one...

You might want to read about the Soviet TOPAZ and TOPAZ II reactors; nuclear reactors in space are basically a solved problem from the technical standpoint. They're just politically impossible, and, to be fair, completely pointless. While in the 1960s and 1970s solar cells had poor enough performance that nuclear reactors made sense to power missions requiring a lot of electrical energy, like the Soviet radar satellites, modern cells are much lighter and more efficient--30% or more efficient for the best commercial products--and see a far greater investment from commercial interests on Earth than nuclear reactors. A reactor would make sense if you were planning on going to Jupiter or beyond, or very close to the Sun, as solar cells do not function very well in either place, but just going to Mars...no. As far as the inner solar system, between Venus and Mars, is concerned, solar cells are basically superior in every way for in-space applications, and only getting better.

The whole idea of reducing risks by working on methods to get to Mars and back impossibly--and I use that word advisedly, because if the articles you link are anything like the Ad Astra mission, they require unrealistically light weight power sources--quickly is inadvisable in any case. Provided that permanent or long-term habitation is undertaken, then the issues your concept has "dodged" will need to be tackled in any case, only they won't have been because of your focus on developing high-energy electrical thrusters and the necessary power systems (which have relatively few other applications, beyond human BEO missions). It is also widely agreed that a Mars mission should last much more than a month or so to avoid being a "flags and footprints" mission with little scientific value, which I can't see how your proposed architecture allows; a round trip of months might only allow a few days on the Martian surface.

I know nothing about aerospace engineering, but why couldn't they create an artificial magnetic field around the capsule to reduce/eliminate the cosmic ray threat to the crew? Is the energy required not feasible?

Uncharged particles (e.g. gamma rays) tend to be the ones that are the most dangerous, because they represent whole-body doses. Those can't be altered by magnetic fields.

Here on Earth, gamma radiation is the most penetrative (I have no fucking idea what you mean by "whole-body doses"), but out in space, it's charged particles going at very fine percentages of the speed of light.

The kind of magnetic field you'd need to deflect those is impressive. Impressive as "wipe your credit cards from seventy million miles" impressive. Imagine Earth's entire magnetic field (which is not nearly enough for this application) is concentrated inside a crewed capsule. That's about one one millionth of what you'd need. Earth cheats, as it gets a hundred miles of atmosphere to soak up what gets through.

We're talking a magnetosphere on par with Jupiter. The magnetosphere you need to generate would dominate the environment in a much greater volume than the Sun's physical volume. It would extend behind you, as sculpted by the solar wind, for fifteen to twenty AU.

And it'd kill any astronauts foolish enough to be on the same spacecraft.

Well, a magic wand to make launching nuclear reactors into space politically acceptable, for one...

I think it could be sold as long as there was a mission worth doing like JIMO that used it.

A space reactor would probably use HEU. It would be less radioactive than the RTGs we already launch until after it went critical, which it would only need to do when we were sure it wasn't coming back. I'm sure there'd be protests, but for the actual environmental impact hearings they'd be able to say (and be correct in saying) that equivalent amounts of material are routinely emitted by coal power plants.

Well, a magic wand to make launching nuclear reactors into space politically acceptable, for one...

I think it could be sold as long as there was a mission worth doing like JIMO that used it.

A space reactor would probably use HEU. It would be less radioactive than the RTGs we already launch until after it went critical, which it would only need to do when we were sure it wasn't coming back. I'm sure there'd be protests, but for the actual environmental impact hearings they'd be able to say (and be correct in saying) that equivalent amounts of material are routinely emitted by coal power plants.

While I agree that all of this is true, it would have to be an extremely worthwhile mission to get funded, and it would have to be human because of the necessary budget. One reason JIMO died was because it was going to cost $20 billion to develop, build, and operate before launch costs (all nicely detailed in a report from NASA I have which unfortunately doesn't seem to be available since NTRS appears to be down at the moment). And they were going to need three or so Delta IV Heavies to do that, so another billion or so dollars. That's a lot of money, way the heck more than SMD could possibly afford--look at how screwed they are with the cheaper JWST or how they couldn't afford the Europa orbiter (which was going to be a mere $5 billion or so).

And as I pointed out, solar cells are actually better, technically, in a lot of ways than nuclear reactors--and, unlike them, getting better all the time because of significant commercial interest, both for Earth and space applications. By the time a Mars mission is actually launched, if it's electrically powered it'll probably be solar electricity. And obviously solar cells have no political problems whatsoever. Now, reactors do have one major niche in inner solar system applications: surface power. They can provide power all day and all night at less weight than a solar cell/regenerative fuel cell system (which has traditionally been the main alternative). That might be enough for them to be developed--most Mars surface mission plans since Zubrin need surface power for fuel production. However...there's always beamed power. Less developed, but it has many of the same advantages as solar cells, including the political advantages.

Well, a magic wand to make launching nuclear reactors into space politically acceptable, for one...

You might want to read about the Soviet TOPAZ and TOPAZ II reactors; nuclear reactors in space are basically a solved problem from the technical standpoint. They're just politically impossible, and, to be fair, completely pointless. While in the 1960s and 1970s solar cells had poor enough performance that nuclear reactors made sense to power missions requiring a lot of electrical energy, like the Soviet radar satellites, modern cells are much lighter and more efficient--30% or more efficient for the best commercial products--and see a far greater investment from commercial interests on Earth than nuclear reactors. A reactor would make sense if you were planning on going to Jupiter or beyond, or very close to the Sun, as solar cells do not function very well in either place, but just going to Mars...no. As far as the inner solar system, between Venus and Mars, is concerned, solar cells are basically superior in every way for in-space applications, and only getting better.

The whole idea of reducing risks by working on methods to get to Mars and back impossibly--and I use that word advisedly, because if the articles you link are anything like the Ad Astra mission, they require unrealistically light weight power sources--quickly is inadvisable in any case. Provided that permanent or long-term habitation is undertaken, then the issues your concept has "dodged" will need to be tackled in any case, only they won't have been because of your focus on developing high-energy electrical thrusters and the necessary power systems (which have relatively few other applications, beyond human BEO missions). It is also widely agreed that a Mars mission should last much more than a month or so to avoid being a "flags and footprints" mission with little scientific value, which I can't see how your proposed architecture allows; a round trip of months might only allow a few days on the Martian surface.

I don't see the political issues with launching reactors, myself. We've been sending up RTGs for ages. A better question would be: can a Billionaire private individual get licensing / approval to purchase the necessary fuel for a TOPAZ type reactor (thanks for the reference... been reading on it)? This thread is, after all, about a private individual and/or conglomerate going to Mars essentially for shits and giggles, not NASA. And getting there *faster* gets around a whole hosts of spam-in-a-can issues that have already been raised in this thread.

Can solar cells provide the MW range power requirements of high-energy thrusters? The admittedly detail-light articles I'm referencing don't indicate so. I'm also not sure what the weight of a 200 N thrust MPDT would be, or it's fuel consumption or fuel weight.

While I agree that all of this is true, it would have to be an extremely worthwhile mission to get funded, and it would have to be human because of the necessary budget. One reason JIMO died was because it was going to cost $20 billion to develop, build, and operate before launch costs (all nicely detailed in a report from NASA I have which unfortunately doesn't seem to be available since NTRS appears to be down at the moment). And they were going to need three or so Delta IV Heavies to do that, so another billion or so dollars. That's a lot of money, way the heck more than SMD could possibly afford--look at how screwed they are with the cheaper JWST or how they couldn't afford the Europa orbiter (which was going to be a mere $5 billion or so).

I was speaking to the political acceptability of launching a reactor.

There's been smaller designs proposed. There's a design being discussed that uses heat pipes/stirling generators to provide as little as 500 watts as an RTG replacement (since Pu-238 is scarce these days).

Budgets are always a problem, but all else being equal, I don't think a reactor would be the stopper.

truth is life wrote:

And as I pointed out, solar cells are actually better, technically, in a lot of ways than nuclear reactors--and, unlike them, getting better all the time because of significant commercial interest, both for Earth and space applications.

Agreed, but there's applications that make sense, outer solar system at a minimum.

NervousEnergy wrote:

Can solar cells provide the MW range power requirements of high-energy thrusters?

It's not so much the power as it is the specific power - power to weight. That determines acceleration.

There's been smaller designs proposed. There's a design being discussed that uses heat pipes/stirling generators to provide as little as 500 watts as an RTG replacement (since Pu-238 is scarce these days).

Budgets are always a problem, but all else being equal, I don't think a reactor would be the stopper.

Well, yeah, I know that smaller designs have been proposed. However, the problem you point out is much less of an issue as of a few days ago; combined with the ASRG and possible larger variants, I think it makes more political and financial sense to avoid using reactors in such low-power applications. Tens of kilowatts or better are where

Megalodon wrote:

Agreed, but there's applications that make sense, outer solar system at a minimum.

Outer, deep inner, and surface are the trinity of useful reactor applications, although those are narrowing somewhat--see the advancement of LILO panels over the past decade or two such that the main limitation on solar-powered spacecraft to Jupiter is the radiation environment. I actually see surface as being the most plausible future reactor deployment site--most surface development proposals need a lot of power, beamed power is very poorly developed at the moment compared to nuclear reactors, and there is a genuine weight advantage over solar/RFC systems. Humans--because of the cost, as I previously mentioned, the only plausible customer of reactors--aren't likely to venture into the outer system in this century (if only because there's fuck-all to go to in any vaguely reasonable time-frame), and sending them into the inner solar system (interior to Venus) would be a pointless waste of time and energy.

Megalodon wrote:

It's not so much the power as it is the specific power - power to weight. That determines acceleration.

Exactly. Solar cells are to the point where the structure is the main problem; apparently (DO NOT have citable sources on this, though) IKAROS demonstrated 1 kWe/kg--that's low enough that you could use the solar cell array as a solar cell to eke out a bit more thrust at no extra weight (which was what IKAROS was, actually). Compare to TOPAZ-II, which I believe is one of the most developed light-weight designs; that would generate 5 kWe at a weight of almost one metric ton. So five times as much power for one thousand times the weight. Of course, a realistic solar power system would need much more weight for the structure, power conversion and control systems, and so forth. But as far as the cells go, the best flown examples are essentially weightless.

Well, yeah, I know that smaller designs have been proposed. However, the problem you point out is much less of an issue as of a few days ago; combined with the ASRG and possible larger variants, I think it makes more political and financial sense to avoid using reactors in such low-power applications.

That's encouraging! I wasn't sure they'd ever get moving on that.

Yeah, the stirling generators are much more efficient. That will definitely increase the capabilities of RTG missions compared to what's gone before.